1. Field of the Invention
This invention relates to a method of repairing cracks in the shaft of a pump for hot liquids, such as a reactor coolant pump, and a pump which has been so repaired.
2. Background Information
Pumps which circulate hot liquids such as the reactor coolant pumps in a pressurized water reactor (PWR) are subjected to harsh conditions. For instance, in the PWR, the reactor coolant water is at a temperature in excess of 500xc2x0 F. and a pressure of about 2250 psi. Cracking was observed in the shafts of pressurized water reactor coolant pumps early in their service life. These cracks were located in the labyrinth region where the shaft penetrates a thermal barrier and were attributed to thermal cycling from the improper mixing of cooler water within the thermal barrier with the hotter primary water in the pump casing. To address this problem, the shaft was fitted with a stainless steel thermal sleeve installed with a light interference fit.
It has been observed that this thermal sleeve has not been effective in preventing shaft cracking. Cracking has also been found on the thermal sleeve and in the labyrinths. Extensive evaluations have shown that under operating conditions a gap opens between the lower end of the sleeve and the shaft due to differential thermal expansion. This in turn has caused thermal oscillations in the sleeve-shaft annulus. These thermal oscillations are aggravated by bypass flow down the openings between the shaft/impeller key and keyway due to the large pressure differential between the top of the impeller and the bottom. The resulting cyclic thermal stresses, in combination with residual tensile stresses in the shaft from initial machining operations, has lead to thermal fatigue cracking in the shaft, thermal sleeve and adjacent labyrinths. Cracks up to about 0.2 inch depth have been measured to date in the shaft and labyrinths, while through-wall cracks have been seen in the thermal sleeve in some instances. The current approach to addressing the cracking problem is to either replace the pump shaft completely, a very expensive solution, or to machine the lower portion of the shaft diameter by turning on a lathe to remove existing cracks and to install a new stainless steel sleeve with a smaller inside diameter. There is a significant consequence of this repair approach. Reducing the shaft diameter near the lower end reduces the existing length of the taper fit engagement between the shaft and the impeller which is already marginal in several pump models. This could lead to excessive stresses on the key, fretting wear of the mating surfaces and result in loosening of the fit and excessive vibrations. There is, therefore, a limit to the depth of cracks that can be repaired by this current method. This limit is about 0.06 inches. Furthermore, the continued use of stainless steel material for the thermal sleeve does not address the problem of gaps opening between the shaft and sleeve under operating conditions.
There is a need therefore for an improved method of repairing cracks in reactor coolant pump shafts and labyrinths and for reactor coolant pumps repaired by such a method.
This need and others are satisfied by the invention which is directed to a method of repairing cracks in a reactor coolant pump shaft adjacent to a tapered end of the shaft on which an impeller is seated which comprises machining a circumferential groove in the pump shaft adjacent the tapered end to a depth and over an axial length to remove the cracks. The circumferential groove however does not extend into the tapered end of the shaft and therefore does not comprise the tapered fit of the impeller on the shaft. A split ring is installed in the circumferential groove. The split ring is sized to fill the groove so that the outer diameter of the split ring is flush with the outer diameter of the shaft. A thermal sleeve is then fit over the split ring. Preferably the thermal sleeve is shrink fit over the split ring and more preferably, has a coefficient of thermal expansion which is less than that of the shaft and of the split ring. The use of a sleeve material with a smaller thermal expansion coefficient than stainless steel ensures that no gaps open between the sleeve and shaft under operating conditions. The split ring preferably has complementarily chamfered axially extending confronting edges which mate.
An annulus is formed between the thermal sleeve, and the shaft and split ring. Preferably this annulus is formed by spaced apart internal ribs integrally formed with the thermal sleeve. The annulus contains substantially stagnant reactor coolant water which is an excellent insulator. Preferably, the annulus communicates with the pump chamber sufficiently that the pressure in the annulus equalizes with the pressure in the pump chamber yet the reactor coolant in the annulus remains substantially stagnant.
A washer between the retainer nut threaded onto a stud on the tapered end of the shaft which retains the impeller in place on the tapered end, extends radially outward to overlap the interface between the impeller and the tapered end of the shaft. This reduces flow of hot reactor coolant in the pressure chamber through this interface and especially around the key which locks the impeller to the shaft.
The invention also embraces a reactor coolant pump which has been repaired in accordance with the method.